Journey
to the centre of the earth

Deep
down inside

The
Earth is alive. Its heart is not just a rock. The Earth lives
its life by spinning around the Solar system, the galaxy and
the Universe.

The
Earth is the platform from which we observe not only the rest
of the universe, but also the Earth itself  even what happens
inside it, said Dr. Michael Rochester, Earth Sciences,
whose research involves a project in the theory of how the Earth
behaves as an entire planet.

It
is only one stage in a series of projects that I have been engaged
in for about 40 years. Since my graduate studies I have been
interested in the ways in which things that we can observe at
the surface of the Earth tell us something about the constitution
of the Earths deep interior, and the processes that go
on inside it.

Over
the last hundred years geophysical scientists have found convincing
evidence that the planet Earth consists of three parts 
solid mantle (extending down to a depth of 2,900 km), fluid outer
core (some 2,300 km thick) and solid inner core (1,200 km radius).
Dr. Rochester is very interested in mostly the fluid outer core
but more recently also the solid inner core  how they affect
the things that we can see on the surface of the Earth.

My
motivation is to understand the dynamics of the Earth,
he said. I am particularly interested in the Earths
fluid core. We cant get a sample of it, we cant drill
that deep but we can infer some of the things it is doing.

For
Dr. Rochester, the Earth is like a gigantic engine and
heat is flowing out from the deep interior.

Fluids
in the outer core transport heat from the inner core to the bottom
of the mantle where it is gradually carried off.

At
the Earths surface the short-term consequences of this
process are earthquakes and volcanoes, while the long-term consequences
are mountain ranges rising and falling, and continents moving
about.

One
of the objects of Dr. Rochesters research is to predict
the enormously small changes in gravity associated with waves
in the fluid core, or with movements of the inner core. Because
it is surrounded by fluid, the solid inner core can move sideways,
back and forth. In principle, an earthquake which is large enough
and deep enough could jiggle the lower boundary of the mantle
sufficiently that the jiggling could be passed on to the inner
core.

To
test how well the theory works, the resulting small periodic
changes in gravity must be detected against the background of
the much larger changes in gravity due to the tides. This detection
may be possible if gravity is measured continuously and very
precisely, for a decade or so, at a large number of places on
the Earths surface, well separated from one another geographically.

The
Global Geodynamics Program (started in 1998) aims at providing
these measurements. It uses very sensitive gravity meters operating
at the very low temperature of liquid helium. The essential part
of such a gravity meter is a little metal ball suspended in a
vacuum. Because the metal becomes a superconductor at these low
temperatures, the ball can be levitated against gravity by a
magnetic field. Any change in gravity can be measured by how
much you have to change the electrical current producing that
magnetic field, in order to offset the tendency of the ball to
rise or fall slightly as gravity changes. There are about 20
superconducting gravity meters deployed around the world. One
is in Canada, near Ottawa.

Because
the iron fluid outer core is a very good metallic conductor of
electricity, it produces and supports the magnetic field of the
Earth. In the core of the Earth the magnetic field is quite strong.
A weak magnetic field affects the North Pole moving it several
metres every hundred years.

Because
the outer core is fluid it becomes possible for the solid mantle
and the solid inner core to each have its own axis of rotation,
slightly offset in direction from one another. Because of their
rotation, the solid mantle and the solid inner core are not spherical
but slightly flattened at their rotation poles and slightly bulging
at their equator, so as to have an ellipsoidal shape. Any changes
in the relative orientation of the mantle and inner core can
be accommodated by the fluid outer core moving so that it continues
to fill the space between them.

It
turns out that there are several ways in which the solid mantle,
fluid outer core and solid inner core can be misaligned. With
each such kind of misalignment the Earth shows a particular kind
of wobbling motion in space, like a gyroscope which has been
disturbed from equilibrium. These wobbles, with their characteristic
periods, can be detected in the gravity records.

But
they are more precisely detected by the associated disturbances
in the locations of distant stellar radio sources relative to
the Earth, as measured by radio telescopes. This technique, called
very-long-baseline-radio-interferometry (VLBI for short), was
actually developed in Canada over 30 years ago. Dr. Rochester
is currently trying to track down the reasons why the theory
of such wobbles predicts one of them to have a period significantly
different from the value deduced from VLBI data.

From
its density and from arguments based on chemical abundance, geophysicists
conclude that the outer core is mostly iron. From seismology
we know it is fluid. Because it is both fluid and a good electrical
conductor, the outer core can produce and sustain the Earths
magnetic field by working like a gigantic dynamo. Although the
magnetic field that reaches the surface of the Earth is weaker
than the field of an ordinary refrigerator magnet, it is nevertheless
responsible for controlling things like a compass needle, and
the Northern Lights.

Dr.
Rochester is interested in how the presence of this magnetic
field changes the rotation of the Earth. As the Earth rotates,
its speed may change because of the changes in the magnetic field.
These very small changes in the Earths rotation speed mean
changes in the length of the day, amounting to a few thousandths
of a second over a time span of a decade. Although so small,
these changes too can be precisely detected using VLBI.

One
of the beautiful things about geophysics to me is that you are
constantly having to bring together different bits of science
in order to understand some phenomenon, said Dr. Rochester.
Nothing stands alone. You cant look at it and understand
it without bringing in knowledge from many other areas. Its
a very interdisciplinary type of field. So many things in science
are connected with one another.

To
my mind, in research you can be interested in something but you
are never quite sure what it is you are going to find. That is
why research is endlessly fascinating.

SPARK,
Students Promoting Awareness about Research Knowledge, is a NSERC
- funded program designed to encourage writing about research.